Prevent the scalarizer from caching incorrect entries

[frasercrmck]

Summary

As described in https://llvm.org/bugs/show_bug.cgi?id=24248, the scalarizer can cache incorrect entries when walking up a chain of insertelement instructions. This occurs when it encounters more than one instruction that it is not actively searching for, as it unconditionally caches every element it finds. The fix is to only cache the first element that it isn't searching for so we don't overwrite correct entries.

[frasercrmck]

Adding Hal as he subscribed to the bug.

[hfinkel]

test/Transforms/Scalarizer/cache-bug.ll

16 ↗

(On Diff #30803)

We don't need to check the entire output here, we only need to check for the function name, this line, and perhaps the absence of the bad output (just to make it clear what is being tested). I'd write this:

; CHECK-LABEL: @func
; CHECK-NOT: phi i32 [ %x, %entry ], [ %inc.pos.y, %loop ]
; CHECK: phi i32 [ %inc, %entry ], [ %inc.pos.y, %loop ]
; CHECK: ret void

The problem with including the whole output verbatim is that any future simplifications, or even changes to how the variables are named, will break the test.

Otherwise, LGTM.

llvm/trunk/lib/Transforms/Scalar/Scalarizer.cpp

if (PtrTy) {
     for (;;) {
       InsertElementInst *Insert = dyn_cast<InsertElementInst>(V);
       if (!Insert)
         break;
       ConstantInt *Idx = dyn_cast<ConstantInt>(Insert->getOperand(2));
       if (!Idx)
         break;
       unsigned J = Idx->getZExtValue();
-      CV[J] = Insert->getOperand(1);
       V = Insert->getOperand(0);
-      if (I == J)
+      if (I == J) {
+        CV[J] = Insert->getOperand(1);
         return CV[J];
+      } else if (!CV[J]) {
+        // Only cache the first entry we find for each index we're not actively
+        // searching for. This prevents us from going too far up the chain and
+        // caching incorrect entries.
+        CV[J] = Insert->getOperand(1);
+      }
     }
     CV[I] = Builder.CreateExtractElement(V, Builder.getInt32(I),
                                          V->getName() + ".i" + Twine(I));
   }
   return CV[I];
 }
 
 bool Scalarizer::doInitialization(Module &M) {

llvm/trunk/test/Transforms/Scalarizer/cache-bug.ll

+; RUN: opt -scalarizer -S < %s | FileCheck %s
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
+
+
+; Check that vector element 1 is scalarized correctly from a chain of
+; insertelement instructions
+define void @func(i32 %x) {
+; CHECK-LABEL: @func(
+; CHECK-NOT: phi i32 [ %x, %entry ], [ %inc.pos.y, %loop ]
+; CHECK:     phi i32 [ %inc, %entry ], [ %inc.pos.y, %loop ]
+; CHECK:   ret void
+entry:
+  %vecinit = insertelement <2 x i32> <i32 0, i32 0>, i32 %x, i32 1
+  %inc = add i32 %x, 1
+  %0 = insertelement <2 x i32> %vecinit, i32 %inc, i32 1
+  br label %loop
+
+loop:
+  %pos = phi <2 x i32> [ %0, %entry ], [ %new.pos.y, %loop ]
+  %i = phi i32 [ 0, %entry ], [ %new.i, %loop ]
+  %pos.y = extractelement <2 x i32> %pos, i32 1
+  %inc.pos.y = add i32 %pos.y, 1
+  %new.pos.y = insertelement <2 x i32> %pos, i32 %inc.pos.y, i32 1
+  %new.i = add i32 %i, 1
+  %cmp2 = icmp slt i32 %new.i, 1
+  br i1 %cmp2, label %loop, label %exit
+
+exit:
+  ret void
+}